The present invention relates generally to the field of AM (amplitude modulation) broadcast transmitters which are to be converted from analog emission to digital transmission as digitalization moves forward, and particularly to a method for reducing out-of-band emission in AM transmitters for digital transmission.
The hitherto usual transmitter types are non-linear AM transmitters which feature an RF (radio frequency) input and an audio input and which are to continue in use. The reasons for this are as follows:                AM transmitters internally operate in switched mode and therefore have efficiencies which are better by a factor of 3 than those of linear transmitters which are otherwise usually used for digital transmission, for example, in the case of DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting). This results in a saving of operating costs.        it is easier to convince broadcasters to migrate from analog to digital if no greater investments come up in the preliminary stages.        
The use of a non-linear AM transmitter for digital modulation requires a special operating mode of the transmitter. In the case of analog AM, only the envelope of the HF oscillation (high frequency) is influenced according to the message signal. If, in place of an audio signal, a digital signal was fed into the modulator, “on-off-keying” (OOK) or something equivalent to “amplitude shift keying” (ASK) would result as the digital modulation. In the vector diagram, OOK or ASK are only on the positive real axis, respectively.
In digital modulations, however, it is usual and, because of the better signal-to-noise ratio, also necessary for the digital signal to pass through the entire complex plane. When looking at the respective sampling points of a digital signal in the vector diagram, the appertaining phase star is obtained. Since the set points thereof are spread over 4 quadrants and do not lie on a straight line as in the case of OOK or ASK, the minimum distances of the set points are larger (given identical energy expenditure for the transmitted digital signal).
The modulated digital signal is generated by two partial signals (I and Q), which are orthogonal to each other. The I signal (“in phase”) is modulated onto a cosine oscillation having the frequency Ft (carrier frequency). The Q signal (“quadrature”) is modulated onto a sine oscillation having the same frequency Ft. The sum of both modulated oscillations produces the complex modulated data signal (cosine 0–180 degrees, sine 90–+90 degrees). The modulated I/Q signal is shaped by filters in such a manner that it has exactly the prescribed curve shape with the desired bandwidth.
However, it is required for the modulated I/Q signal to be converted in such a manner that the two signals amplitude signal (A signal) and phase-modulated carrier signal (RF-P) result therefrom which are suitable for proper control of the AM transmitter (see FIG. 2). Then, at the output of the AM transmitter, the modulated I/Q signal is generated again with higher power.
The modulated I/Q signal corresponds to a Cartesian representation. The Cartesian representation is converted to a polar representation with amplitude and phase. In this manner, the amplitude signal (A signal) is obtained to control the AM transmitter at the audio input. A phase-modulated radio frequency (RF-P signal) is generated from the initially resulting phase signal (P signal). Advantageously, the RF-P signal can also be directly obtained without the intermediate step via the P signal. In this manner, the signals are generated that are required for controlling the AM transmitter:                amplitude signal (A signal) for controlling the audio input        phase-modulated RF signal (RF-P signal) for controlling the RF input.        
The A signal is fed into the modulator input (audio input) of the AM transmitter, and the RF-P signal is used for HF-type control of the transmitter. In the transmitter output stage, the two signals A & RF-P are multiplicatively combined, forming the high frequency digital output signal. The output signal is theoretically identical to the correspondingly amplified complex modulated I/Q signal prior to the coordinate conversion.
The above-described use of AM transmitters is described, for example, in European Patent EP 0 708 545 and in German Patent DE 197 17 169.